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The exciton dynamics in InxGa1−xAs/GaAs quantum wells grown on (111)B and (100) GaAs substrates were studied by time-resolved photoluminescence (PL) under magnetic fields in a Faraday configuration. We have found that the piezoelectric fields in (111)B samples affect the transient behavior of the PL spectra. Compared with the reference (100) sample, we have found that the strong piezoelectric fields, as well as the magnetic fields, cause a slower spin–flip process in (111)B strained quantum wells.

Radiative and non-radiative recombination processes of excitons in quantum dots of diluted magnetic semiconductors were studied in magnetic fields. The Cd1−xMnxSe quantum dots fabricated by the self-organized mode on ZnSe showed marked increases of the photoluminescence intensity and the decay time in the magnetic field. These phenomena are related to the magnetic field induced shrinkage of the exciton Bohr radius.

We report an experimental evidence of the existence of free excitonic magnetic polaron (FEMP) states from the study of the dynamics of free and bound excitons in Cd 1−x Mn x Te (x=0.03) by time-resolved photoluminescence measurements. We found, for the first time, that FEMP are more stable than the polarons formed from bound excitons. The polaron binding energy for the free excitons is evaluated to be 4 meV , which is considerably larger than that of 1 meV found for the magnetic polarons of the bound excitons; these values were determined from the suppression of the excitonic energy relaxation in high magnetic fields. As a consequence, the peak energies of the optical emissions of free excitons and donor-bound excitons cross in the time-resolved photoluminescence spectra.

We carried out time-resolved magneto-photoluminescence experiments in diluted magnetic semiconducting Cd1−xMnxTe MBE grown epilayers to study the excitonic localization mechanisms. We found that, for samples with a high non-radiative recombination rate, the luminescence intensity and the decay time decrease significantly under a magnetic field. This shows that the magnetic localization dominates over the non-magnetic localization when magnetic polarons are formed.

Nanometer-scale structures of II-VI diluted magnetic semiconductors were synthesized by the microfabrication techniques. Quantum dots of Cd 1 -x Mn x Se (x = 0.03) with diameters of 9.0-9.4 nm were grown by the self-organized mode in the molecular beam epitaxy. Quantum wires of Cd 1 -x Mn x Se (x = 0.08) were synthesized with the width of 60-125 nm from the two-dimensional quantum wells by the electron beam lithography and chemical etching. The optical properties and the exciton dynamics of these quantum structures were studied by the transient photoluminescence spectroscopy. The magneto-optical properties of the confined excitons in the quantum dots, the wires and the wells are discussed.

Time-resolved photoluminescence has been studied on molecular beam epitaxy grown Cd 1− x Mn x Te epilayers and Cd 1− x Mn x Te/Cd 1− y Mg y Te single- and multiple-quantum wells. The formation of exciton magnetic polarons is detected in the single-quantum wells of various well widths. The transient photoluminescence of magnetic polarons is significantly affected by the external field. The photoluminescence intensity of the exciton is found to show two-exponential decay characteristics. The magnetic field dependence of the two decay times is attributed to the conversion of the dark excitons to the bright excitons by a spin-flip process. The temperature dependence of exciton decay times is interpreted as the thermal activation process in the multiple-quantum well.

We report on a field-dependent photoluminescence (PL) emission rate for the transitions between band states in modulation-doped CdTe/Cd1−xMgxTe single quantum wells in the integer quantum Hall region. The recombination time observed for the magneto-PL spectra varies in concomitance with the integer quantum Hall plateaus. Furthermore, different PL decay times were observed for the two circular polarizations, i.e. for the transitions between the Zeeman split subbands of the Landau levels. We analyzed the data in comparison with the experimentally determined spin polarization of the conduction electrons and the Zeeman splitting of the valence band. Furthermore, we discuss the relevance of the spin polarization of the conduction electrons, the electron–hole exchange interaction and the spin-flip processes of the hole states for the PL decay time.

We present a study of the exciton dynamics in modulation doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ heterojunctions and GaAs epilayers. The comparison permits to identify the features characteristic for the heterojunctions. In particular, we analyze the rise time of the transient photoluminescence (PL) intensity. In general, we find a longer PL rise time for the lower-energy excitons indicating that the time required for the energy relaxation process increases with increasing binding energy of the excitons. Moreover, the rise time of the free excitons turns out to be conspicuously longer in heterojunctions than in epilayers although the time integrated PL spectra of the two systems are similar. From our analysis we conclude that the long rise time observed in the heterojunctions is closely connected with the vertical drift of the photoexcited carriers driven by the interface potential. In fact, we find that the carriers (i.e., holes in n-doped samples) photoexcited in the heterojunctions drift vertically from the heterointerface to the flat band region, where they finally recombine after forming excitons (bimolecular formation) with oppositely charged and locally excited carriers (i.e., electrons in n-doped samples).

Exciton dynamics has been studied in quantum wells, quantum dots and quantum wires of diluted magnetic semiconductors by time-resolved photoluminescence spectroscopy. The transient exciton photoluminescence with resonant excitation in Cd 1− x Mn x Te/ZnTe ( x =0.1) multiple quantum wells shows the formation of magnetic polarons in the quantum well excitons. Free and localized states of the excitonic magnetic polarons are distinguished from the difference of the lifetimes and the binding energies. The magnetic polaron formation accompanied with emission or absorption of LO phonons was also observed. Quantum dots of Cd 1− x Mn x Se grown by the self-organization mode showed the luminescence of confined excitons in the dots. The Mn ions in the quantum dots induce large Zeeman shifts and the excitonic magnetic polaron formation in the dots. Quantum wires of Cd 1− x Mn x Se showed polarized exciton emissions parallel to the wires. The polarization was decreased by the external magnetic field perpendicular to the wires. The excitonic magnetic polaron states in the quantum wires are also discussed.

The exciton dynamics in In 0.15 Ga 0.85 As/GaAs quantum wells grown on (111)B and (100) GaAs substrates are studied by the time-resolved photoluminescence (PL). We have found that the piezoelectric fields in (111)B samples affect the transient behavior of PL spectra. Compared with the reference (100) samples, we have confirmed that the piezoelectric effect induces slower exciton relaxation in (111)B strained quantum wells.

Employing two different growth methods: standard molecular beam epitaxy (MBE) and low-temperature atomic layer epitaxy (ALE) with subsequent annealing, we have obtained high-quality quantum dot structures consisting of CdSe embedded in ZnSe. Single dot emission lines are observed in micro-luminescence. The samples have been investigated by further optical methods including time-resolved photoluminescence under resonant excitation at 4.2 K. Distinct properties of systems with three-dimensional confinement are observed such as the suppression of the interaction between isolated quantum dots (QDs). In standard quantum wells tunneling/hopping processes generally lead to a pronounced red shift of the luminescence over time due to a lateral localization of excitons in potential fluctuations. A much less pronounced red shift is observed for the QDs reflecting only the different lifetimes of single dots and higher excited states. The red shift completely vanishes under resonant excitation that selectively excites only a few QDs of the ensemble in the layer. Typical behaviour is also observed from the halfwidth of the quantum dot emission.

The exciton dynamics in Ga1 − xInxAs/GaAs self-organized quantum dots grown on GaAs (111)B substrates are studied by the time-resolved photoluminescence (PL). We have found the intra-dot exciton relaxation by the reduction of the linewidth and peak energy and also by the energy-dependent PL rise time in the transient PL spectra. Compared with the energy relaxation in the reference quantum wells, we have confirmed that the exciton relaxation in three-dimensionally confined quantum dots is slower than in the quantum wells.

We present a study of the dynamics of the excitonic magnetic polarons in Cd1−xMnxTe epilayers. The transient photoluminescence measured by a streak camera under resonant excitation is analyzed with a special attention devoted to line shape of the emission signal. A nonradiative spectral region is observed in zero field and is strongly suppressed in a field of 6 T. This observation is understood to be directly related to the alignment of the Mn2+ spins and is therefore closely related to the formation of magnetic polarons. A few Mn spins are found to be aligned already within 10 ps, but the total Mn spin polarization in the sample is found to increase exponentially with a rise time of 37 ps in Cd0.73Mn0.27Te epilayers. Furthermore, the values of 19.2 and 12.4 meV were determined for the energies of the excitonic magnetic polarons in Cd0.73Mn0.27Te and Cd0.79Mn0.21Te epilayers, respectively.

We report optical properties of quantum dots and quantum wires of diluted magnetic semiconductors. The quantum dots of Cd1−xMnxSe (x=0.03) show the exciton luminescence at around 2.4 eV, which indicates a strong confinement effect of the exciton energy corresponding to the dot size of 4–6 nm. The Zeeman shift of the exciton luminescence was observed with an effective g value of 91, showing a significant exchange interaction of the excitons with the Mn ions in the dots. The exciton luminescence from the quantum wires of Cd1−xMnxSe (x=0.08) shifts by 5.2 meV to the higher energy side with decreasing the wire width from 126 to 26 nm. The high energy shift in the narrow wires indicates the influence of the one-dimensional quantum confinement effect for the exciton states. The effective g value of the exciton in these quantum wires is 100–150. The exciton luminescence from the wires is linearly polarized (up to 80%) parallel to the wire direction at zero field, which indicates one-dimensional properties of the...

Time-resolved photoluminescence (PL) and optical pump–probe experiments were carried out in Cd1−xMnxTe/ZnTe multiple quantum wells to study the dynamics of the exciton magnetic polaron in this material. In the pump–probe data, three signals were detected. The main signal is negative and monitors the energy relaxation of the excitons created by the pump pulse. The other two pump–probe signals are positive and their intensity grows while the excitons created by the pump pulse recombine. The low-energy positive pump–probe signal is found to track the average Mn spin polarization in the sample. On the contrary, the high-energy positive peak indicates the enhancement of the density of states due to correlation effects.

Exciton localization is studied with time-resolved photoluminescence (PL) in Cd 1− x Mg x Te epilayers. The energy relaxation rate of excitons is found to be energy dependent and reduces drastically below an effective excitonic mobility edge. This particular property results in the saturation of the PL intensity rise time below the mobility edge and a two-exponential decay for the PL intensity and the energy relaxation of the excitons.

The lifetime of excitons in Cd1−xMnxTe/Cd1−yMgyTe single quantum wells was investigated by time-resolved photoluminescence spectroscopy. Spin-flips between dark and bright excitons remarkably change the excitonic lifetime yielding a two-exponential decay in the photoluminescence intensity. The temperature dependence of the two decay times shows that the spin-flip rate depends strongly on the formation of excitonic magnetic polarons. By forming polarons, the spin-flip rate from dark excitons to bright excitons is reduced effectively by the spin- barriers of the polarized Mn ions, which extends significantly the luminescence decay time.

Luminescence of excitons has been studied in quantum nanostructures of Cd1−xMnxSe by time-resolved spectroscopy with femtosecond pulse lasers. Self-organized quantum dots show luminescence of quantum-confined excitons. Quantum wires show the exciton luminescence polarized parallel to the wires. Large Zeeman effects and the excitonic magnetic polaron formation are observed in these quantum structures.

The decay properties of excitons in Cd 1− x Mn x Te/Cd 1− y Mg y Te single and multiple quantum wells are investigated by the time-resolved photoluminescence measurements. We observed the conversion of the dark excitons to the bright excitons by a spin-flip process. This yields a two-exponential decay of the photoluminescence intensity. The magnetic field dependence of the two-exponential decay is discussed in the frame of the spin-flip scattering and the giant Zeeman splitting in single quantum wells. The temperature dependence of exciton decays is interpreted by the increase of exciton scattering rate due to thermal activation in multiple quantum well.

Time-resolved photoluminescence has been measured on molecular beam epitaxy grown Cd1−xMnxTe epilayer samples. The photoluminescence intensity of magnetic polarons exhibits a very long emission tail. The observed phenomenon is attributed to the conversion of the dark excitons to dipole allowed transitions by a spin-flip process. The magnetic field dependence of the two-exponential decay property is discussed within the frame of giant Zeeman splitting, the spin-flip scattering, and the formation of the magnetic polarons.

Optical pump-probe experiments were performed in Cd0.9Mn0.1Te/ZnTe multiple quantum wells to study the dynamics of the exciton magnetic polaron in this material. Two transient absorptions of opposite sign were observed. The first signal is negative and indicates the decrease of optical absorption, as the excitons are approaching saturation. The second signal is positive and is interpreted as the absorption of “new excitons” after the initial excitons created by the pump pulse have decayed leaving behind a magnetic polarized Mn environment. The spin relaxation time of the Mn ions has been determined to be 601 ps. Finally, we discuss the effect of the polaron–polaron interaction, relevant in our experiment because we saturate the excitons in the quantum wells.

We investigated the dynamics of free and bound excitonic magnetic polarons in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ $(x=0.03)$ single crystals by time-resolved photoluminescence experiments. We found that the magnetic polarons formed from free excitons are more stable than those formed from bound excitons. As a consequence, the peak energies of the optical emissions of free excitons and donor-bound excitons cross in the time-resolved photoluminescence spectra. Furthermore, we observed that nonmagnetic localization is not required to form free magnetic polarons.

We verify the validity of the optical characterization of the 2D electron gas confined in modulation doped (GaAl)/As/GaAs heterojunctions designed for both high electron mobility devices. The properties of the 2D electron gas are evaluated from the magneto photoluminescence and the microwave influenced photoluminescence detected on the GaAs excitons in the flat band region. We evaluate the electron density and mobility from the oscillation of the photoluminescence intensity in a magnetic field, and the effective mass from the resonant change of the PL intensity under additional microwave radiation. We attribute these non-linear optical properties to the vertical transport and the bimolecular exciton formation of photoexcited carriers. We proved these specific transport and exciton formation processes by time-resolved photoluminescence experiments, where an extremely long exciton formation time and a strong signal of anti-stokes emissions under resonant excitations are observed. Furthermore, we realized a significant amplification of the photoluminescence intensity by exciting the heterojunction with two temporarily delayed laser pulses. The amplification of the luminescence intensity reflects unambiguously the recycling through the bimolecular exciton formation of the photo-excited carriers accumulated in the flat band region after vertical transport.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.